This invention relates to a magnetic bubble memory device and in particular to a hybrid type magnetic bubble memory device using both ion-implanted propagation tracks and permalloy propagation tracks, in which the ion-implanted propagation tracks are suitable for reducing the size of the device.
A device storing binary information by using magnetic bubbles or magnetic domains in a magnetic thin film (for example, made of magnetic garnet) whose axis of easy magnetization is perpendicular to the film surface attracts attention. In particular, an ion-implanted magnetic bubble memory device disclosed in U.S. Pat. No. 3,828,329 has an advantage that the operation characteristics are excellent even if the period of bubble propagation tracks is made small.
However, when function portions of the magnetic bubble memory device are formed by using ion-implanted propagation tracks instead of permalloy devices, the characteristics of some portions are not good. The characteristics of generators and detectors formed by ion-implanted propagation tracks are relatively good, but the characteristics of replicate gates and swap gates formed by ion-implanted propagation tracks are very bad.
Therefore, a hybrid type magnetic bubble memory device in which minor loops using ion-implanted propagation tracks are combined with permalloy propagation tracks has been proposed (JP-A-57-40791). In this device, the characteristics of a function portion using permalloy propagation tracks are improved by making the period of the permalloy propagation track 3-6 times as great as that of the ion-implanted propagation tracks in the minor loop.
Further, it has been also proposed to make the storage density in the minor loop portion higher by constructing the minor loop portion with ion-implanted propagation tracks which are several times folded, as shown in FIG. 1, because the period of the permalloy propagation tracks is greater than that of the ion-implanted propagation tracks. In the construction shown in FIG. 1, each of minor loops 1 is connected with read and write major lines 2 and 3 through replicate and swap gates 4 and 5 which are made of permalloy patterns. In FIG. 1, reference numeral 6 represents a bubble generator and numeral 7 represents a bubble detector.
A concrete structure of a minor loop in the hybrid type bubble memory device is shown in FIG. 2. Areas indicated by oblique lines in the figure represent ion-implanted regions. As shown in the figure, a central portion of the minor loop is formed by an ion-implanted propagation track 10 while upper and lower end portions thereof (at the neighborhood of the corners) are formed by permalloy propagation tracks 11. In order to prevent the decrease in a bubble drive force at the boundary of each junction 12 between the ion-implanted propagation track 10 and the permalloy propagation tracks 11, bubbles are transferred at the junction 12 by a combined bubble drive force of both the ion-implanted pattern and the permalloy pattern. For that purpose, it is necessary that a position at which an attracting magnetic pole exists due to the ion-implanted pattern and a position at which an attracting magnetic pole exists due to the permalloy pattern coincide with each other for the arbitrary phase of a rotating field. Therefore, as shown in FIG. 2, the ion-implanted pattern 13 and the permalloy pattern 14 are superposed on each other. In this case, since the period of this permalloy pattern is 3-6 times as great as that of the ion-implanted propagation track at the central portion of the minor loop, as mentioned above, the period of the ion-implanted propagation track 10 at the junction 12 is also 3-6 times as great as that of the ion-implanted propagation track 10 at the central portion of the minor loop.
In the past, such two ion-implanted propagation tracks 10 having remarkably different periods were connected so that recessed portions seen from the non ion-implanted region, i.e. so-called cusps may coincide with each other, as shown in FIG. 3a. However, in the structure shown in FIG. 3a, the operation margin of the whole propagation tracks is narrow, as shown in FIG. 4c. Therefore, this structure is not put into practical use. FIG. 4a shows a relation between the rotating field and the bias field for ion-implanted propagation tracks consisting of a disk pattern having a period of 3 .mu.m, and FIG. 4b, a relation between the rotating field and the bias field for ion-implanted propagation tracks consisting of a disk pattern having a period of 10 .mu.m.
The reason why the operation margin of the whole device with the structure indicated in FIG. 3a is narrow is because when bubbles are propagated from the disk pattern having a greater period to the disk pattern having a smaller period, there takes place a phenomenon in which bubbles propagated to the smaller-period disk pattern are pulled apart therefrom by strong magnetic poles of the greater-period disk pattern. This phenomenon appears remarkably when the phase of the rotating magnetic field is in a direction indicated by arrows in FIG. 3a. When the rotating field is at the phase indicated in FIG. 3a, properly the bubble should be at a position indicated by 50 in the figure. However, when the phase of the rotating field is at such a phase, (+) poles attracting a bubble as indicated in the figure are produced in the disk pattern having the greater period. Then, in the disk pattern having the greater period, the large amount of these attractive poles due to its large period gives rise to an erroneous operation by which a bubble, which is attracted by the disk pattern having the smaller period and is at a position indicated by 50, is pulled apart therefrom and jumps to a position indicated by 51. The situation described above is identical for the case, where the shape of the ion-implanted propagation track at the boundary of an ionimplanted region is not of disk type but of square type, as shown in FIG. 3b.
In order to prevent such an erroneous operation, a structure has been proposed in which the period of the ion-implanted propagation track is gradually increased with an increased distance from the central portion of the minor loop toward the junction 12 between connecting the ion-implanted propagation track portion having the greater period and the ion-implanted propagation track portion having the smaller period, as shown at an area 15 in FIG. 2. As clearly seen in FIG. 2, in the device having such a structure, the central portion of a minor loop has a higher bit density since three contiguous disk patterns are provided by folding the minor loop. However, it is not possible to fold the minor loop at a portion 15 thereof where the period is gradually increased. This is because the period of the minor loop at such a portion 15 is greater than that of the central portion. Consequently, the bit density of the minor loop portion 15 in a direction perpendicular to the bubble propagation direction is one third of that of the central portion of the minor loop. Also in the bubble propagation direction, the bit density of the minor loop portion 15 is smaller than that of the central portion of the minor loop.
Nowadays, the storage density in the magnetic bubble memory device becomes rapidly higher. However, there was a problem that the portion where the period is increased is a useless area in a chip, thereby preventing realization of a high storage density of the device.